Integrated tunable fabry-perot filter and method of making same

ABSTRACT

A tunable Fabry-Perot filter includes an optical cavity bounded by a stationary reflector and a deformable or movable membrane reflector. A second electrostatic cavity outside of the optical cavity includes a pair of electrodes, one of which is mechanically coupled to the movable membrane reflector. A voltage applied to the electrodes across the electrostatic cavity causes deflection of the membrane, thereby changing the length of the optical cavity and tuning the filter. The filter with the movable membrane can be formed by micro device photolithographic and fabrication processes from a semiconductor material in an integrated device structure. The membrane can include an inner movable membrane portion connected within an outer body portion by a pattern of tethers. The tether pattern can be such that straight or radial tethers connect the inner membrane with the outer body. Alternatively, a tether pattern with tethers arranged in a substantially spiral pattern can be used.

RELATED APPLICATION

This application is based on U.S. Provisional Patent Application Ser.No. 60/186,780, filed on Mar. 3, 2000.

BACKGROUND OF THE INVENTION

A Fabry-Perot filter (FPF) is an optical device which is constructed topass light of a selected band of wavelengths. Light entering the filterenters a cavity which is bounded by a pair of reflective surfaces. Thereflective surfaces are separated by a precisely controlled distancewhich determines a set of passbands for the filter. The smaller theseparation, the further apart the passbands are in wavelength. That is,the smaller the separation, the larger the free spectral range (FSR) ofthe filter.

A tunable FPF adds an adjustable component to the separation by whichthe peak wavelengths of the passbands can be changed. Typically, tuningis achieved in a miniature FPF by making one of the two reflectors amovable or deformable membrane and applying a voltage between themembrane and the second fixed reflector, thereby changing the cavityseparation distance through electrostatic attraction. In such a device,the amount of deflection and, therefore, cavity length control, isdependent upon the distance between the reflectors and the level of theapplied voltage. For a given starting separation, more deflectionrequires a higher voltage level; and, likewise, for a given voltagerange, more deflection requires that the reflectors be closer together.

At voltage levels compatible with smaller miniature devices, the priorapproach to tuning FPFs restricts the device to a relatively smallcavity size. This constraint can greatly inhibit the performance of thedevice by restricting control over the wavelength passbands.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a tunable filter, which inone embodiment is a Fabry-Perot filter (FPF), and a method whichovercome these drawbacks of the prior approaches. The FPF of theinvention includes an input by which light enters the filter and anoutput by which filtered light exits the device. A first cavity, e.g.,the optical cavity, is provided between the input and the output and isbounded by first and second reflective surfaces. As with theconventional prior art FPF, the wavelengths of light exiting the filterare dependent upon the length of the first cavity. In the presentinvention, at least one of the reflective surfaces is movable to varythe length of the first cavity to tune the device. The invention alsoprovides a second cavity which is outside the first cavity. A voltagecan be applied across the second cavity to move the movable reflectivesurface to change the length of the first cavity and thereby tune thefilter.

In one embodiment, the electrostatic cavity has two electrodes, onefixed and one movable. The movable electrode is coupled to the movablereflective surface. The voltage is applied across the electrostaticcavity via the two electrodes to move the movable surface, therebychanging the length of the first or optical cavity on the opposite sideof the movable surface to tune the filter.

One of the reflective surfaces can be curved to present a concave shapeto the inside of the first (optical) cavity. The curved reflector can beproduced by a mass transport (MT) process of the type described in U.S.Pat. No. 5,618,474, by Liau, et al., issued Apr. 8, 1997, entitled,“Method of Forming Curved Surfaces by Etching and Thermal Processing,”the contents of which are incorporated herein in their entirety byreference. The concave mirror significantly loosens the angularalignment tolerances on the device, and the MT fabrication approachproduces an extremely smooth surface, which is beneficial for highoptical finesse on the micro-lens-scale concave surface.

The movable reflective surface can be formed as a movable membranenested within an outer body. The movable membrane is connected to theouter body by a plurality of flexible tethers or flexures. The movablemembrane moves axially with respect to the outer body portion via theflexing or deformation of the flexures under the electric field appliedacross the electrostatic cavity. The flexures are shaped and sized toprovide a desired amount of deflection under expected voltage ranges andoptical operational characteristics. In one particular embodiment theflexures extend between the movable membrane and the outer body in astraight or radial pattern. In another embodiment, the flexures areformed in a substantially spiral pattern. This latter configurationprovides longer flexure length and, therefore, more deflection underapplied voltage, while maintaining a relatively small overall surfacesize.

This reflective surface with the movable membrane, outer body portionand tether pattern can be formed using semiconductor device fabricationtechniques. For example, the tether, membrane and body patterns can bedefined on a semiconductor layer such as a silicon wafer byphotolithography. The patterns can then be formed in the semiconductorby one or more etching steps. The movable membrane can then be at leastpartially coated with a high reflectivity (HR) coating to provide thedesired reflective characteristics for the interior of the opticalcavity.

In one embodiment, the filter, e.g., FPF, of the invention is anintegrated structure fabricated using semiconductor device fabricationand photolithographic techniques. In one particular embodiment, thedevice is formed from a silicon-on-insulator device structure.

In the integrated structure of the invention, a first reflective layerand a second reflective layer are formed spaced apart by a spacing layerinterposed between them. The thickness of the spacing layer determinesthe distance between the reflective layers and, therefore, the length ofthe first, i.e., optical, cavity of the device. At least one of thereflective layers comprises the movable reflective surface noted abovewhich makes the filter tunable. An electrode layer is disposed spacedapart from the movable reflective layer to define the second(electrostatic) cavity outside of the first (optical) cavity. Thevoltage is applied across the second cavity to move the movablereflective layer.

In one embodiment, the device includes a first electrode coupled to theelectrode layer and a second electrode coupled to the movable reflectivelayer. The voltage used to move the movable layer is applied across thesecond cavity via the electrodes.

In one embodiment, the spacing layer includes a semiconductor layer,such as a silicon layer. The silicon layer can be sized such as bygrinding and polishing to a precise thickness to control the length ofthe optical cavity of the device. In another embodiment, the spacinglayer includes a layer of oxide grown or deposited on a semiconductorlayer. The thickness of the oxide can be used to control the length ofthe cavity. In one embodiment, the spacing layer includes a layer ofsemiconductor and a layer of oxide, the thickness of either or both ofwhich can be controlled to control the length of the cavity. In anotherembodiment, the spacing layer comprises a plurality of spacing posts,which can be attached to one or both of the reflective layers. The postscan be made of metal such as gold and can be plated or bonded to one orboth of the reflective layers.

The tunable filter and method provide numerous advantages over theapproaches of the prior art. For example, as described above, in thepresent invention, the optical cavity is not the same as theelectrostatic cavity. Therefore, the length of the optical cavity can bedefined independently of the relationship between the required membranedeflection and the applied deflection voltage. As a result, the cavitycan be designed with the freedom and flexibility to meet only specificoptical requirements without the electrical constraints introduced inthe prior art devices. The filter's optical parameters, e.g., freespectral range, and the electrostatic tuning parameters can beindependently optimized. A much more precise device with more desirableoptical performance as well as more efficient electrical performance isobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 contains a schematic cross-sectional diagram of one embodiment ofan optical filter in accordance with the invention.

FIG. 2 is a schematic plot illustrating an exemplary relationship of theseparation distance between an electrode and a movable membrane versusthe net attractive/repulsive electrostatic force on the membrane, forseveral applied voltages, in accordance with the invention.

FIGS. 3A through 3G are schematic plan views of various configurationsof the movable membrane layer with different tether patterns inaccordance with the invention.

FIGS. 4A through 4I contain schematic cross-sectional views illustratingfabrication of one embodiment of the filter of the invention.

FIGS. 5A through 5G contain schematic cross-sectional views illustratingfabrication of another embodiment of the filter of the invention.

FIGS. 6A through 6F contain schematic cross-sectional views illustratingfabrication of another embodiment of the filter of the invention.

FIGS. 7A through 7G contain schematic cross-sectional views illustratingfabrication of another embodiment of the filter of the invention.

FIGS. 8A through 8C contain schematic cross-sectional views illustratingan approach to forming spacing posts for defining the length of theoptical cavity in the filter according to the present invention.

FIGS. 9A through 9D contain schematic cross-sectional views illustratinganother approach to forming spacing posts for defining the length of theoptical cavity in the filter according to the present invention.

FIG. 10 contains a schematic plot of wavelength versus gain illustratingthe performance of one embodiment of the improved tunable FPF of theinvention.

FIGS. 11A through 11C contain perspective pictorial images of threetypes of tether and membrane configurations in accordance with thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 contains a schematic cross-sectional view of one embodiment of atunable Fabry-Perot filter (FPF) 10 in accordance with the invention.The filter 10 includes three main functional layers, including a tuningdrive electrode 12, a moving membrane reflector 14, and a concave, e.g.,spherical, cavity reflector 16. These functional layers are heldtogether and operated as a tunable FPF with several interstitial layers,as described hereinafter in detail.

The reflectors 16 and 14 define the optical cavity 18 of the device 10,which in this case has a concave shape due to the concave shape of thereflecting surface of the reflector 16. The curved reflector can beformed in accordance with the approach described in U.S. Pat. No.5,618,474, incorporated herein by reference above. The length of thecavity 18 is changed to tune the filter 10 by applying a voltage acrossthe moving membrane reflector electrode 14 and the tuning driveelectrode 12 using the adjustable voltage source 22. Upon application ofthe tuning voltage, an electric field is generated in the electrostaticdrive spacing or cavity 20. The electrostatic forces cause the membraneto deflect, thus altering the length of the optical cavity 18 as,desired. FIG. 2 is a schematic plot illustrating an exemplaryrelationship of the separation distance between the electrode 12 andmembrane 14 versus the net attractive/repulsive electrostatic force onthe membrane 14, for several applied voltages. A tuning range isillustrated in which electrode/membrane contact is avoided.

In accordance with the invention, the reflective membrane layer 14 ismade to deflect under the applied voltage to change the cavity lengthand thereby tune the filter 10. FIGS. 3A through 3G are schematic planviews of various configurations of the movable membrane layer 14 inaccordance with the invention. As shown in FIGS. 3A through 3G, eachmembrane layer 14 includes an outer body portion 36 and an innermembrane portion 34. The membrane portion 34 is supported in the outerbody portion 36 by a pattern of tethers or flexures 32. In FIGS. 3Athough 3G, the layers 14 can be primarily distinguished by their tetherpatterns. FIG. 3A illustrates a straight or radial tether pattern withsix radial tethers 32A. FIG. 3B illustrates a straight or radial tetherpattern with five tethers 32B. FIG. 3C illustrates a “loop” tetherconfiguration in which tethers feed or loop back on themselves in thearea between the outer body 36C and the movable membrane 34C. FIG. 3Dillustrates a dogleg spiral tether pattern, referred to as a “thin”spiral because the tethers 32D are relatively thin. FIG. 3E illustratesa “thick” dogleg spiral pattern with relatively thick tethers 32E. FIG.3F illustrates a nested spiral pattern in which the tethers 32F can beextended to overlap each other. FIG. 3G illustrates a nested swepttether design in which the tethers 32G are extended to overlap eachother. That is, the length of the tethers is such that a radial lineextending from the center of the membrane portion 34G could intersectmultiple, e.g., two, tethers 32G.

It is noted that in the loop and spiral tether patterns of FIGS. 3Cthrough 3G, longer tethers 32 are realized in the same overall devicesize. This can help provide desired membrane deflection performancewithout having to increase the size of the device. Formicro-electro-mechanical system (MEMS)-based devices, such as the filterof the present invention, using a deflectable membrane structure, it isdesirable to optimize the stiffness of the supporting tethers to meet agiven set of performance requirements. By using a spiral tether design,the diameter of the inner membrane portion 34 can be made larger for anequivalent length radial tether (holding the outside diameter fixed).This provides a proportionally larger area over which the electrostaticforces can act, thereby reducing the voltage requirement of the device.

The effective area of the radial tether designs can be increased byadding “paddles” to the central membrane. These are appendages thatextend outward from the inner membrane in regions between the tethers.The paddles extending from the central membrane can introduceundesirable vibrational modes to the structure, however. Additionalconstraints are imposed on the design process by the limitations ofavailable fabrication methods. For example, if a designer is required todecrease the stiffness of the structure while maintaining a constantmembrane and tether thickness, there are two options: 1) increase thetether length; or 2) decrease the tether width. It is usually moredesirable to increase the tether length since the stiffness of a beam ismore sensitive to changes in length. However, for a straight tethergeometry, this results in an overall increase in device size, which mayviolate a performance requirement. While decreasing the width of atether will reduce its stiffness, the lower limit that can be achievedwill be imposed by limitations in the fabrication process. Formicro-fabricated devices, the minimum tether width is determined byminimum resolvable feature size that can be produced using the availablephotolithography and/or etching processes. As the tether widthapproaches this minimum, the structural response of the system willbecome more sensitive to process variations, making it difficult toachieve high yield in manufacturing.

A spiral tether geometry is one in which the tether originates from thecentral membrane at an angle oblique to the local normal. The tethercontinues along a trajectory such that it intersects the outercircumference of the device at a similar angle, although the originatingand terminating angles need not be equal. This is distinct from astraight tether design in which the tethers originate from the centralmembrane in a purely radial direction and likewise intersect the outercircumference. In the case of spiral geometries, such as those shown inFIGS. 3D through 3G, by orienting the tethers at an oblique angle,longer tethers are possible for a given overall device size. Thus, amembrane structure with a given spring constant, i.e., stiffness, can beachieved within a smaller overall diameter. Furthermore, with a spiralgeometry, the tethers can be nested together, as shown in FIGS. 3F and3G. That is, the originating point of one tether can occur at anazimuthal angle less than that of the point of termination of theneighboring tether. This can be seen in FIG. 3G proceedingcounter-clockwise around the pattern and assuming that tethers 32Goriginate on the inner membrane portion 34G and terminate on the outerbody portion 36G. This feature allows the tether length to be variedlover a wider range of values, thus providing greater freedom of design.Since the length can be increased to a larger value, the width of thetether can be maintained at a value well above the resolution of thefabrication process, thus improving the manufacturability of the device.

An additional performance advantage of the spiral tether geometry isthat it has reduced response to stresses transferred from the centralmembrane area, compared to straight tether designs. For the tunablefilter of the invention, a (HR) coating is applied to the centralmembrane to form a moving mirror. Residual stress in this coatinginduces curvature in the central membrane. Experimental measurements andfinite element analyses have shown that the resulting deflection ofspiral tethers is less than that of straight tethers. This improvedimmunity to residual stress in the high reflectivity coating improvesthe manufacturing yield of the devices. In accordance with theinvention, spiral tether geometries are developed to minimize theinduced deflection.

Various processes for fabricating the filter of the invention will nowbe described in detail. FIGS. 4A through 4I contain schematiccross-sectional views illustrating fabrication of one embodiment of thefilter of the invention. In this embodiment, referred to as “OxideDefined Electrostatic Drive,” the assembly starts with a base wafer,Wafer A, which in one embodiment is produced from a standard n-typedoped silicon wafer and which serves as the supporting substrate for theentire device. Wafer A is typically 75 mm to 150 mm in diameter and is400 to 500 microns thick. The wafer A is oxidized to a depth x₁,typically 2 to 4 microns and which is specified to achieve the designparameters for the electrostatic tuning drive. The maximum tuning rangeis approximately 33% of this oxide thickness (see FIG. 2), and therequired maximum tuning voltage is inversely proportional to the squareof the thickness, as is typical for electrostatic drives.

As shown in FIG. 4B, a second n-type doped silicon wafer, Wafer B, isbonded to Wafer A using elevated temperature and mechanical pressure.Wafer B, which will become the electrostatically deflectable siliconmembrane, is ground to a thickness t₁, typically 6 to 10 microns. Aftergrinding, the surface of Wafer B is oxidized to a thickness x₂,typically 0.5 to 1.0 micron. As shown in FIG. 4C, a membrane and tetherpattern, such as those shown in FIGS. 3A through 3G, is etched into theoxide grown on Wafer B.

Next, as shown in FIG. 4D, a third n-type doped silicon wafer, Wafer C,is bonded to the oxide on Wafer B, again using elevated temperature andmechanical pressure. This wafer, Wafer C, will become the cavity spacerthat defines the optical properties of the FPF. Wafer C buries themembrane-patterned oxide on Wafer B and is subsequently ground to athickness t₂, typically 15 to 25 microns, that is appropriate to themirror-to-mirror spacing of the curved mirror-flat mirror Fabry-Perotoptical cavity. This translates to a free spectral range of 45 to 80 nm.

As shown in FIG. 4E, an optical port 101 is patterned and etched intoWafer A using a combination of isotropic and anisotropic etching. Theoxide layer x₁ is used as an etch stop. Alternatively, the optical portetch step can be omitted, as silicon is partially transparent atinfrared wavelengths, in which case an anti-reflective (AR) coating isapplied to the outer surface of Wafer A to minimize reflection from theair-silicon interface. With the process described, the underside of themembrane and opposing side of the handle wafer will not have AR. Thus,the optical performance of the device could be compromised. A spacer andelectrical contact pattern 103 is etched into Wafer C using the oxide ofthickness x₂ as an etch-stop layer. As shown in FIG. 4F, a silicon etchis performed anisotropically to transfer the diaphragm and tetherpattern into the underlying silicon (Wafer B), using the oxide ofthickness x₁ as an etch stop layer. One approach would be to usedirectional reactive ion etching for this step.

As shown in FIG. 4G, the resulting structure is subjected to anisotropic oxide etchant to “release” the membrane and tether structurefrom the oxide layer x₁, and the etch-stop oxide is removed from theopenings forming the spacer and contact. In one embodiment, this wouldcall for the use of concentrated HF followed by methanol, followed by adrying step using supercritical carbon dioxide.

As shown in FIG. 4H, a high reflectivity (HR) multi-layer dielectricmirror 105 is deposited through the spacer opening onto the membraneinterior surface using an appropriate shadow mask. An anti-reflection(AR) coating 107 is similarly deposited through the optical port ontothe exterior surface of the membrane. Both of these coatings aredesigned for the wavelength bands of interest.

Next, as shown in FIG. 4I, electrical contacts 111 and 113 are depositedon the back side of Wafer A and in the contact opening of Wafer B,respectively, in one embodiment using aluminum or a refractory metal.Next, as shown in FIG. 4I, a concave, highly polished micro-mirror 117is installed on top of the spacer layer. In one embodiment, the mirror117 is made in accordance with U.S. Pat. No. 5,618,474, incorporatedherein by reference. The mirror 117 has an appropriate HR coating 15 onits interior surface and an appropriate AR coating 109 on its exteriorsurface, so that it forms a precision, high-finesse optical cavity inconjunction with the diaphragm or membrane. High parallelism andaccurate spacing is maintained because of the uniformity of the spacergrinding thickness. The mirror attachment can be performed usinggold-tin attachment layers (or Au/AuSn spacers in the alternateimplementation without the spacer wafer attached) formed by depositionof plating. If a deposited or plated spacer is used, the metalcomposition may be graded so that eutectic melting occurs only near theattachment interface to the mirror.

FIGS. 5A through 5G contain schematic cross-sectional views illustratingfabrication of another embodiment of the filter of the invention. Inthis embodiment, referred to as “Silicon Defined Electrostatic Drive,”the wafer assembly again starts with the base wafer, Wafer A, which willbecome the supporting substrate for the entire device, as shown in FIG.5A. In one embodiment, Wafer A is 75 mm to 150 mm in diameter and 400 to500 microns thick. The wafer is oxidized, or receives deposited oxidewhich is subsequently densified, to a thickness x₁, typically 0.5 to 1.0micron. A second n-type doped silicon wafer, Wafer B, is bonded to WaferA using elevated temperature and mechanical pressure. Wafer B, which inthis embodiment will become the spacer between the membrane and thedrive electrode, is ground to a thickness t₁, which can be 1 to 3.5microns. As shown in FIG. 5B, after grinding, Wafer B is patterned andetched to form part of the cavity 119 between the mirror membrane andelectrode. During this etch, the oxide layer of thickness x₁ is used asan etch stop. Alternatively, the cavity etch can be omitted andperformed at a later step.

Referring to FIG. 5C, setting aside the A-B structure, a separate,400-500 microns thick, n-type doped silicon wafer, Wafer C, of the samesize as Wafers A and B, is oxidized, or receives deposited oxide whichis subsequently densified, to a thickness x₂, which can be 0.5 to 1.0micron. Wafer C is patterned and the oxide thickness x₂ is etched toform a deflectable mirror membrane and tether pattern. Another n-typedoped silicon wafer, Wafer D, is bonded to Wafer C. Wafer D is ground toa thickness t₁ suitable for an electrostatically deflectable siliconmembrane thickness, which can be 6 to 10 microns. As shown in FIG. 5D,Wafer C/D assembly is flipped over, and the ground surface of Wafer D isbonded to the oxidized surface of Wafer B using elevated temperature andmechanical pressure. Wafer C is subsequently ground to a thickness t₂,which can be 15 to 25 microns, appropriate to the mirror-to-mirrorspacing of the curved mirror-flat mirror Fabry-Perot optical cavity.

Referring to FIG. 5E, an optical port 101 is patterned and etched intoWafer A, using a combination of isotropic and anisotropic etching. Theoxide of thickness x₁ is used as an etch stop. If the cavity etch wasnot performed in an earlier step, the oxide of thickness x₁ is removed,and the cavity is formed by isotropic etching. As in the embodimentdescribed above in connection with FIGS. 4A through 4I, the optical portetch step can be omitted. If the optical port is omitted, ananti-reflective (AR) coating is applied to the outer surface of Wafer Ato minimize reflection from the air-silicon interface.

As shown in FIG. 5E, a spacer and electrical contact pattern is etchedinto Wafer C, using the oxide x₂ as an etch stop layer. This is followedby an anisotropic silicon etch to transfer the diaphragm or membrane andtether pattern from the oxide x₂ into the underlying silicon (oxide x₁is the etch stop layer). In one implementation, directional reactive ionetching is used for this step. Next, as shown in FIG. 5F, the etch stopoxides are removed from the openings, releasing the diaphragm andtethers without wet etchant, thus avoiding the static friction(“stiction”) caused by liquid surface tension.

The remaining steps are similar to those of the embodiment describedabove in connection with FIGS. 4A though 4I. A HR mirror layer 105 isdeposited through an appropriate shadow mask and the spacer opening ontothe membrane surface, and an AR coating 107 is deposited on the opticalport side of the membrane. Electrical contacts 111 and 113 are depositedon the surfaces of Wafer A and Wafer D. The contacts 111 and 113 can bemade of aluminum or a refractory metal.

Referring to FIG. 5G, a curved mirror 117 with an HR coating 115 andpatterned metallization, e.g., Ti—Au, 0.5 micron thickness, is attachedto the spacer layer, establishing a precision gap between the curvedmirror surface and the HR coating 105 on the membrane. High parallelismis maintained because of the uniformity of the spacer grindingthickness. The mirror attachment can be performed using AuSn attachmentlayers (or Au/AuSn) formed by deposition of plating. If a deposited orplated spacer is used, the metal composition may be graded so thateutectic melting occurs only near the attachment interface to themirror.

FIGS. 6A through 6G contain schematic cross-sectional views illustratingfabrication of another embodiment of the filter of the invention. Inthis embodiment, referred to as “Plated Airbridge,” the assembly startswith a standard n-type doped silicon wafer, Wafer A, which in oneembodiment is 75 mm to 150 mm in diameter and 400 to 500 microns thick.In this embodiment, Wafer A is used as the cavity spacer for theFabry-Perot cavity, a dielectric membrane is the moving mirror, and aplated electrode formed by an “airbridge” technique is the fixed driveelectrode.

Referring to FIG. 6A, the process starts with application of a HRcoating on the top surface of Wafer A, followed by an AR coating, at athickness of 3 to 9 microns, on the HR coating. This resultingdielectric membrane 141 will become the moving membrane of the filter asdescribed below. In one embodiment, the HR/AR coating 141 includesalternating layers of dielectric material such as SiO_(x) or TiO_(x)with the thicknesses and deposition conditions adjusted to leave a nettensile stress in the dielectric film.

The membrane electrode pattern 145 is created next. A resist layer isapplied, exposed and patterned using, for example, an image reversaltechnique, to achieve resist undercut. Next, electrode metal, such asTi—Pt—Au or Ti—Ni—Au, of total thickness of 0.5 micron is deposited. Themetal is then lifted off to leave patterned membrane electrode metal,metal traces and bonding pads. Alternatively, the electrode metal isdeposited first, patterned with resist, then etched to form the membraneelectrode metal.

Referring now to FIG. 6B, next, another layer of resist 147, the“airbridge” resist, is applied, exposed and patterned on the surface ofthe HR/AR coating layer 141 and electrode pattern 145. This resist 147is used to protect the membrane electrodes and define the gap betweenthe membrane electrodes and the fixed electrodes, which are depositednext. In order to protect the existing membrane electrodes and properlyform the fixed electrodes, the patterned resist 147 is reflowed.

Referring to FIG. 6C, a plating base layer 149, which can be a 0.5micron thick sputtered TI—Au layer, is deposited over the patternedresist and in the resist openings. Next, a thick layer of resist or PMMA151 is deposited on or mechanically attached to the plating base layer149. In one embodiment, the thickness of this resist or PMMA 151 is 20to 600 microns. The thick resist or PMMA 151 is exposed with UV, DUV orsoft X-rays (for example, synchrotron-generated X-rays, as in the LIGAtechnique) then developed to form the plating pattern. Next, referringto FIG. 6D, attaching one electrode to the plating base layer, the waferis electroplated with Au, Ni, NiFe or other suitable plating metal 153.Plating thickness can be as much as the plating resist or PMMAthickness. In one embodiment, the plating thickness is 20 to 600microns. This plated layer 153 forms airbridge electrodes facing themembrane electrodes, an optical port and an integral mounting structure.

Referring to FIG. 6E, next, the thick resist or PMMA is removed,followed by removal of the unplated, sputtered plating base layer 149,using ion milling or Au etchant. The exposed airbridge resist is removedusing isotropic dry etching with heating in a fluorinated oxygen plasma.

In one implementation, the plated integral mounting structure isdesigned to attach to a surface perpendicular to the optical cavity axisusing AuSn solder.

Referring to FIG. 6F, using the plated structure for support, Wafer A isground to a thickness t₂, which in one embodiment is 15 to 25 microns,appropriate to the mirror-to-mirror spacing of the curved mirror-flatmirror Fabry-Perot cavity. A spacer opening 155 is patterned and etchedinto Wafer A, using a combination of isotropic and anisotropic etching.The HR/AR layer 141 is used as an etch stop. The HR/AR layer 141 isreleased at this point, such that it now simultaneously mechanical anoptical functions. The mechanical function is the support of thedeflectable membrane over the spacer opening, and the optical functionis the HR/AR capabilities.

Referring to FIG. 6F, the curved mirror 117 is then attached as in theprevious embodiments.

In the embodiments of the invention described thus far, the length ofthe optical cavity, i.e., the spacing between the curved reflector andthe membrane, is controlled by the thickness of a silicon and/or anoxide layer. FIGS. 7A through 7G contain schematic cross-sectional viewsillustrating fabrication of another embodiment of the filter of theinvention, in which the spacing and, therefore, the length of theoptical cavity can be more precisely controlled by the thickness of oneor more metal layers which are plated or bonded to either the surface ofthe layer in which the membrane is formed or the surface of the curvedmirror structure.

Referring to FIGS. 7A through 7G, it is noted that the steps illustratedin FIGS. 7A through 7C are the same as those illustrated and describedabove in connection with FIGS. 4A through 4C. Therefore, description ofthem will be omitted. In FIG. 7D, Wafer B is etched to form the membraneand tether pattern in the wafer. In FIG. 7E, the remaining oxides on topof Wafer B are etched away isotropically. In FIG. 7F, the HR coating 105and AR coating 107 are applied to the membrane and the electrodes 111and 113 are applied to Wafer A and Wafer B, respectively, as in theprevious embodiments. In FIG. 7G, the mirror assembly 217 is mounted onthe top of Wafer B. Spacing posts 171 are interposed between the top ofWafer B and the mirror assembly 217 to control the cavity length betweenthe mirrors.

FIGS. 8A through 8C contain schematic cross-sectional views illustratingan approach to forming the spacing posts 171 illustrated in FIG. 7G. InFIG. 8A, the mirror assembly 217 includes an AR coating 221 on itsbottom and sides and a HR coating 219 on its top surfaces as shown. Ametal seed layer 223 is sputtered onto the edges of the curved portionof the mirror surface as shown.

Next, in FIG. 8B, a layer of photoresist 225 is applied and patterned todefine the location and size of the spacing posts 171. The metal spacers171 are then formed by plating the exposed seed layer with a metal suchas gold. The metal is plated to a thickness suitable for the cavitylength of the device. In one embodiment, the thickness is 5 to 25microns. Referring to FIG. 8C, the photoresist is removed, leaving themirror assembly 217 with the spacing posts 171.

FIGS. 9A through 9D contain schematic cross-sectional views illustratinganother approach to forming the spacing posts 171 illustrated in FIG.7G. As shown in FIG. 9A, a sacrificial bonding layer made of, forexample, titanium, copper and/or tungsten, is formed on a surrogatesubstrate 231 and patterned, leaving pads 233 on which the spacing posts171 will be formed.

Next, as shown in FIG. 9B, a photoresist pattern 235 is deposited andpatterned as shown to define the location and size of the spacing posts.The posts are then formed by plating metal such as gold onto the pads233 to a thickness appropriate for the desired cavity length. In oneembodiment, the posts 171 are plated to a height of 15 to 25 microns.Next, as shown in FIG. 9C, the substrate 231 with the posts 171 isflipped, and the posts 171 are bonded to bonding metal pads 237 made of,for example, gold, formed on the mirror assembly 217. Next, as shown inFIG. 9D, the surrogate substrate 231 and plating seed pads 233 areremoved, leaving the tops of the spacing posts 171 ready for bonding tothe remainder of the device, as described above.

It should be noted that any of the bonding steps described herein can beperformed by a thermo-compression bonding process. Under that process,the surfaces being bonded together are formed with a multiple conductorstructure which includes a titanium adhesion layer formed on the surfaceof the device to be bonded. A platinum diffusion barrier layer is formedover the adhesion layer, and the gold bonding layer is formed over thebarrier layer. The two structures are then attached at their bondinglayers, and the composite device is subjected to elevated temperature,e.g., 320 degrees C., and pressure, e.g., 200 psi, for a predeterminedperiod of time, which in one embodiment is approximately three minutes.The high pressure and heat cause the structures to bond together.

It is also noted that when the tuning voltage is applied to theelectrodes of the invention, it is possible that the membrane maydeflect sufficiently to make contact with other device surfaces. Forexample, referring to FIG. 4I, it is possible for the bottom side of themembrane formed in Wafer B to make contact with the top surface of WaferA across the electrostatic cavity. After the surfaces touch, it can bedifficult to separate them, even after the voltage is removed, due tostatic friction or stiction between the two surfaces. To solve thisproblem, in one embodiment of the invention, one or both of the surfacescan be roughened to reduce the amount of surface area in contact. In oneembodiment, the roughening is performed by an etching and/or polishingprocess to the original wafer or wafers before assembly. For example,the top surface of Wafer A can be subjected to the roughening process,such as by plasma etching or buffered HF. In one embodiment, rougheningto 5 to 20 nm is used. Alternatively, fluorocarbon films are applied toone or both surfaces.

FIG. 10 contains a schematic plot of wavelength versus gain illustratingthe performance of one embodiment of the improved tunable FPF of theinvention. The plot illustrates the peak wavelength in the passband ofthe filter. The peak is relatively narrow in wavelength compared to thepeaks obtained by FPFs tunable in accordance with conventional methods.

FIGS. 11A through 11C contain perspective pictorial images of threetypes of tether and membrane configurations in accordance with thepresent invention. FIGS. 11A and 11B illustrate two radial or straighttether configurations. FIG. 11C illustrates the loop configurationdepicted in plan view in FIG. 3C.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the following claims.

What is claimed is:
 1. A tunable optical filter comprising: a firstreflective layer; a second reflective layer; a spacing layer between thefirst and second reflective layers defining a first cavity disposedbetween the first and second reflective layers, a thickness of thespacing layer defining a length of the first cavity, and a wavelength offiltered light exiting the filter being related to the length of thefirst cavity, at least one of the first and second reflective layersbeing movable to change the length of the first cavity; and an electrodelayer disposed spaced apart from both of the first and second reflectivelayers to define a second cavity external to the first cavity, a voltagebeing applicable across the second cavity to move the at least one ofthe first and second reflective layers, an optical port being a holeformed through the electrode layer, an optical axis of said filterextending through said first cavity, and said first and second cavitiesbeing arranged at different ordinates along said optical axis; whereinthe at least one of the first and second reflective layers comprises amembrane layer comprising: an outer body portion; and an inner movablemembrane portion within the outer body portion, the inner movablemembrane portion being movable along an axis of the inner movablemembrane portion, said voltage being applicable between the electrodelayer and the membrane layer.
 2. The filter of claim 1 furthercomprising a first electrode coupled to the electrode layer and a secondelectrode coupled to the at least one of the first and second reflectivelayers, the voltage being applicable across the first and secondelectrodes.
 3. The filter of claim 1 wherein the filter is a Fabry-Perotfilter.
 4. The filter of claim 1 wherein at least one of the first andsecond reflective layers comprises a curved surface.
 5. The filter ofclaim 1 wherein at least one of the first and second reflective layerscomprises a concave surface.
 6. The filter of claim 1 wherein theflexures are formed in a substantially radial pattern between the outerbody portion and the inner movable membrane portion.
 7. The filter ofclaim 1 wherein flexures are formed in a substantially spiral patternbetween the outer body portion and the inner movable membrane portion.8. The filter of claim 1 wherein the filter comprises asilicon-on-insulator structure.
 9. The filter of claim 1 wherein thespacing layer comprises a semiconductor layer.
 10. The filter of claim 9wherein the spacing layer comprises an oxide layer.
 11. The filter ofclaim 1 wherein the spacing layer comprises an oxide layer.
 12. Thefilter of claim 1 wherein the spacing layer comprises a plurality ofposts on at least one of the first and second reflective layers.
 13. Thefilter of claim 12 wherein the posts comprise metal.
 14. The filter ofclaim 13 wherein the metal comprises gold.
 15. The filter of claim 13wherein the posts are bonded onto the one of the first and secondreflective layers.
 16. The filter of claim 13 wherein the posts areplated onto the one of the first and second reflective layers.
 17. Thefilter of claim 1 wherein at least one of the first and secondreflective layers comprises a reflective coating.
 18. The filter ofclaim 17 wherein the reflective coating is applied over a semiconductorlayer.
 19. The filter of claim 1 further comprising a second spacinglayer between the electrode layer and the one of the first and secondreflective layers, the spacing layer defining a length of the secondcavity.
 20. The filter of claim 19 wherein the second spacing layercomprises a semiconductor layer.
 21. The filter of claim 20 wherein thesecond spacing layer comprises an oxide layer.
 22. The filter of claim19 wherein the second spacing layer comprises an oxide layer.
 23. Amethod of making an optical filter comprising: forming a firstreflective layer; forming a second reflective layer; forming a spacinglayer between the first and second reflective layers, said spacing layerdefining a first cavity disposed between the first and second reflectivelayers, a thickness of the spacing layer defining a length of the firstcavity, and a wavelength of filtered light exiting the filter beingrelated to the length of the first cavity, at least one of the first andsecond reflective layers being movable to change the length of the firstcavity; and forming an electrode layer disposed spaced apart from bothof the first and second reflective layers to define a second cavityexternal to the first cavity, such that a voltage is applicable acrossthe second cavity to move the at least one of the first and secondreflective layers, an optical axis of said filter extending through saidfirst cavity, and said first and second cavities being arranged atdifferent ordinates along said optical axis; forming an optical porthole through the electrode layer; wherein forming the at least one ofthe first and second reflective layers comprises: forming an outer bodyportion; forming an inner movable membrane portion within the outer bodyportion, the inner movable membrane portion being movable along an axisof the inner movable membrane portion, said voltage being applicablebetween the electrode layer and the movable membrane portion.
 24. Themethod of claim 23 further comprising: forming a first electrode coupledto the electrode layer; and forming a second electrode coupled to the atleast one of the first and second reflective layers such that thevoltage is applicable across the first and second electrodes.
 25. Themethod of claim 23 wherein the filter is a Fabry-Perot filter.
 26. Themethod of claim 23 wherein at least one of the first and secondreflective layers is formed to include a curved surface.
 27. The methodof claim 23 wherein at least one of the first and second reflectivelayers is formed to include a concave surface.
 28. The method of claim23 wherein flexures are formed in a substantially radial pattern betweenthe outer body portion and the inner movable membrane portion.
 29. Themethod of claim 23 wherein flexures are formed in a substantially spiralpattern between the outer body portion and the inner movable membraneportion.
 30. The method of claim 23 wherein the method comprises forminga silicon-on-insulator structure.
 31. The method of claim 23 whereinforming the spacing layer comprises forming a semiconductor layer. 32.The method of claim 31 wherein forming the spacing layer comprisesforming an oxide layer.
 33. The method of claim 23 wherein forming thespacing layer comprises forming an oxide layer.
 34. The method of claim23 wherein forming the spacing layer comprises forming a plurality ofposts on at least one of the first and second reflective layers.
 35. Themethod of claim 34 wherein the posts are formed of metal.
 36. The methodof claim 35 wherein the metal comprises gold.
 37. The method of claim 34wherein forming the posts comprises bonding the posts onto the one ofthe firsthand second reflective layers.
 38. The method of claim 34wherein forming the posts comprises plating the posts onto the one ofthe first and second reflective layers.
 39. The method of claim 23wherein forming at least one of the first and second reflective layerscomprises forming a reflective coating.
 40. The method of claim 39further comprising applying the reflective coating over a semiconductorlayer.
 41. The method of claim 23 further comprising forming a secondspacing layer between the electrode layer and the one of the first andsecond reflective layers, the spacing layer defining a length of thesecond cavity.
 42. The method of claim 41 wherein forming the secondspacing layer comprises forming a semiconductor layer.
 43. The method ofclaim 42 wherein forming the second spacing layer comprises forming anoxide layer.
 44. The method of claim 41 wherein forming the secondspacing layer comprises forming an oxide layer.